older CD transport vs newer ones.

"And I'm not trying to say that a better transport won't be better in some circumstances, but this seems like one of those things that has a very quickly reached point of diminishing returns."

Some may feel that way. I believe that the design, engineering,  built construction,  power supply quality and implementation are just as meaningful for a CD transport as any other electronic/audio component
But as I noted above,  for some the transport for whatever reason is viewed as not as important as the DAC.

Some are of the belief that a "decent" CD player or transport is enough to get the job done. In my listening experiences,  the better the CD transport,  the better the sound quality. 
Charles 

I use a Theta Compli for years now.
Think it was about $5K in the '90s. Probably could not be made today for less than $10K. Theta was at the cutting edge of digital. 

I had Mark Levinson No.37 back in the day, used Phillips Pro transport, laser mech. Best CD transport ever owned, sad day when died, no subsequent transport came close. Gave up on cd plays, exclusive streaming today.

I went through this journey about 2 months ago and can share a bit.
First let me say that i'm not an engineer and please do your own research this is merely what I was able to pick up after combing through a few audio science / social sites.

Do you want to be able to play SACDs?
There is so little content available in SACD and just because it is available on SACD doesn't mean it automatically sounds transcendent. Example: Miles Davis Bitches Brew sound mildly better than the CD box set, however Carlos Santana's Love Devotion Surrender sounds night and day different from the cd. And yes it is transcendent. Now some may argue that it could be that it came from a better master and really has nothing to do with the format. But even if that were true, the SACD is the only format you can get that better master on so... Point is, if playing SACD is important to you the selection process just leaned up a bit. Jays cd transport does not play SACDs.

Jitter
From what I was able to read jitter is more an inherent issue with how a transport connects to the next component in the chain  i.e. coax, optical, etc. That is not to say that other factors within the player and DAC can't be contributors to jitter however all the connections listed have the capacity for jitter therefore it is possible to have jitter. The fix for this is i2s, the way data travels using this mechanism is uniquely different and thus jitter is a non issue. Jitter isn't reduced, it's just not possible. I apologize I was unable to locate the audio science thread that talked about this in depth.

If this is a must for you, you will also need a DAC with compatible i2s. Apparently there are some compatibility issues you will also need to navigate. Schiit is working on a cd transport and mentioned they will not be including i2s because they don't like the compatibility issues.

Upsampling
Is upsampling important? Can you add data where there isn't any? Yes, via interpolation.
https://www.youtube.com/watch?v=SdPU2TZylSs

Where I ended my journey:
SACD: I did want the ability to play SACDs, I know there is a tiny amount of content available but when you are able to find something you love and its mastered really well it does sound amazing.
Jitter: I have a Chord Qutest that I love but sadly it does not have i2s so that need kind of fell off the table for me.
Upsampling: I did want this ability.
I ended up getting a used Cary Audio SACD 306 (2006) that I am very happy with. I use it as a transport when playing cds and go balanced when playing SACDs. It has a bunch of Burr Brown chips in it so its DAC is no slack.

If I were to ever push for an upgrade I would prolly go with the PS Audio Perfect Wave used.

Hope this helps!

 

"If I were to ever push for an upgrade I would prolly go with the PS Audio Perfect Wave used"

The P.S. Audio PWT has stood the test of time. I've been very happily using it in my system the past 8 or 9 years.  Fellow PWT owners who have purchased or heard the Pro-Ject RS2 Box transport report it surpasses the PWT. This is high praise in my opinion as the PWT is an excellent sounding unit. 
Charles